RFC 7496

Internet Engineering Task Force (IETF) M. Tuexen
Request for Comments: 7496 Muenster Univ. of Appl. Sciences
Category: Standards Track R. Seggelmann
ISSN: 2070-1721 Metafinanz Informationssysteme GmbH
R. Stewart
Netflix, Inc.
S. Loreto
Ericsson
April 2015 Additional Policies for the Partially Reliable
Stream Control Transmission Protocol Extension
Abstract
This document defines two additional policies for the Partially
Reliable Stream Control Transmission Protocol (PR-SCTP) extension.
These policies allow limitation of the number of retransmissions and
prioritization of user messages for more efficient usage of the send
buffer.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7496.

1. Introduction
The Partially Reliable SCTP (PR-SCTP) extension defined in [RFC3758]
provides a generic method for senders to abandon user messages. The
decision to abandon a user message is sender side only, and the exact
condition is called a "PR-SCTP policy" ([RFC3758] refers to them as
"PR-SCTP Services"). [RFC3758] also defines one particular PR-SCTP
policy, called "Timed Reliability". This allows the sender to
specify a timeout for a user message after which the SCTP stack
abandons the user message.
This document specifies the following two additional PR-SCTP
policies:
Limited Retransmission Policy: Allows limitation of the number of
retransmissions.
Priority Policy: Allows removal of lower-priority messages if space
for higher-priority messages is needed in the send buffer.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Additional PR-SCTP Policies
This section defines two new PR-SCTP policies, one in each
subsection.
Please note that it is REQUIRED to implement [RFC3758], if you want
to implement these additional policies. However, these additional
policies are OPTIONAL when implementing [RFC3758].
3.1. Limited Retransmissions Policy
Using the Limited Retransmission Policy allows the sender of a user
message to specify an upper limit for the number of retransmissions
for each DATA chunk of the given user messages. The sender MUST
abandon a user message if the number of retransmissions of any of the
DATA chunks of the user message would exceed the provided limit. The
sender MUST perform all other actions required for processing the
retransmission event, such as adapting the congestion window and the
retransmission timeout. Please note that the number of
retransmissions includes both fast and timer-based retransmissions.

The sender MAY limit the number of retransmissions to 0. This will
result in abandoning the message when it would get retransmitted for
the first time. The use of this setting provides a service similar
to UDP, which also does not perform any retransmissions.
Please note that using this policy does not affect the handling of
the thresholds 'Association.Max.Retrans' and 'Path.Max.Retrans' as
specified in Section 8 of [RFC4960].
The WebRTC protocol stack (see [DATA-CHAN]) is an example of where
the Limited Retransmissions Policy is used.
3.2. Priority Policy
Using the Priority Policy allows the sender of a user message to
specify a priority. When storing a user message in the send buffer
while there is not enough available space, the SCTP stack at the
sender side MAY abandon other user message(s) of the same SCTP
association (with the same or a different stream) with a priority
lower than the provided one. User messages sent reliably are
considered to have a priority higher than all messages sent with the
Priority Policy. The algorithm for selecting the message(s) being
abandoned is implementation specific.
After lower-priority messages have been abandoned, high-priority
messages can be transferred without the send call blocking (if used
in blocking mode) or the send call failing (if used in non-blocking
mode).
The IP Flow Information Export (IPFIX) protocol stack (see [RFC7011])
is an example of where the Priority Policy can be used. Template
records would be sent with full reliability, while flow records
related to billing, security, and other monitoring would be sent
using the Priority Policy with varying priority. The priority of
security-related flow records would be set higher than the priority
of monitoring-related flow records.
4. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to support the newly defined PR-SCTP policies, to provide
some statistical information, and to control the negotiation of the
PR-SCTP extension during the SCTP association setup.
Please note that this section is informational only.

4.1. Data Types
This section uses data types from [IEEE.1003-1G.1997]: uintN_t means
an unsigned integer of exactly N bits (e.g., uint16_t). This is the
same as in [RFC6458].
4.2. Support for Added PR-SCTP Policies
As defined in [RFC6458], the PR-SCTP policy is specified and
configured by using the following sctp_prinfo structure:
struct sctp_prinfo {
uint16_t pr_policy;
uint32_t pr_value;
};
When the Limited Retransmission Policy described in Section 3.1 is
used, pr_policy has the value SCTP_PR_SCTP_RTX and the number of
retransmissions is given in pr_value.
When using the Priority Policy described in Section 3.2, pr_policy
has the value SCTP_PR_SCTP_PRIO. The priority is given in pr_value.
The value of zero is the highest priority, and larger numbers in
pr_value denote lower priorities.
The following table summarizes the possible parameter settings
defined in [RFC6458] and this document:
+-------------------+---------------------------+---------------+
| pr_policy | pr_value | Specification |
+-------------------+---------------------------+---------------+
| SCTP_PR_SCTP_NONE | Ignored | [RFC6458] |
| SCTP_PR_SCTP_TTL | Lifetime in ms | [RFC6458] |
| SCTP_PR_SCTP_RTX | Number of retransmissions | Section 3.1 |
| SCTP_PR_SCTP_PRIO | Priority | Section 3.2 |
+-------------------+---------------------------+---------------+

4.3. Socket Option for Getting the Stream-Specific PR-SCTP Status
(SCTP_PR_STREAM_STATUS)
This socket option uses IPPROTO_SCTP as its level and
SCTP_PR_STREAM_STATUS as its name. It can only be used with
getsockopt() but not with setsockopt(). The socket option value uses
the following structure:
struct sctp_prstatus {
sctp_assoc_t sprstat_assoc_id;
uint16_t sprstat_sid;
uint16_t sprstat_policy;
uint64_t sprstat_abandoned_unsent;
uint64_t sprstat_abandoned_sent;
};
sprstat_assoc_id: This parameter is ignored for one-to-one style
sockets. For one-to-many style sockets, this parameter indicates
for which association the user wants the information. It is an
error to use SCTP_{CURRENT|ALL|FUTURE}_ASSOC in sprstat_assoc_id.
sprstat_sid: This parameter indicates for which outgoing SCTP stream
the user wants the information.
sprstat_policy: This parameter indicates for which PR-SCTP policy
the user wants the information. It is an error to use
SCTP_PR_SCTP_NONE in sprstat_policy. If SCTP_PR_SCTP_ALL is used,
the counters provided are aggregated over all supported policies.
sprstat_abandoned_unsent: The number of user messages that have been
abandoned using the policy specified in sprstat_policy on the
stream specified in sprstat_sid for the association specified by
sprstat_assoc_id, before any part of the user message could be
sent.
sprstat_abandoned_sent: The number of user messages that have been
abandoned using the policy specified in sprstat_policy on the
stream specified in sprstat_sid for the association specified by
sprstat_assoc_id, after a part of the user message has been sent.
There are separate counters for unsent and sent user messages because
the SCTP_SEND_FAILED_EVENT supports a similar differentiation.
Please note that an abandoned large user message requiring SCTP-level
fragmentation is reported in the sprstat_abandoned_sent counter as
soon as at least one fragment of it has been sent. Therefore, each
abandoned user message is counted in either sprstat_abandoned_unsent
or sprstat_abandoned_sent.

If more detailed information about abandoned user messages is
required, the subscription to the SCTP_SEND_FAILED_EVENT is
recommended. Please note that some implementations might choose not
to support this option, since it increases the resources needed for
an outgoing SCTP stream. For the same reasons, some implementations
might only support using SCTP_PR_SCTP_ALL in sprstat_policy.
sctp_opt_info() needs to be extended to support
SCTP_PR_STREAM_STATUS.
4.4. Socket Option for Getting the Association-Specific PR-SCTP Status
(SCTP_PR_ASSOC_STATUS)
This socket option uses IPPROTO_SCTP as its level and
SCTP_PR_ASSOC_STATUS as its name. It can only be used with
getsockopt(), but not with setsockopt(). The socket option value
uses the same structure as described in Section 4.3:
struct sctp_prstatus {
sctp_assoc_t sprstat_assoc_id;
uint16_t sprstat_sid;
uint16_t sprstat_policy;
uint64_t sprstat_abandoned_unsent;
uint64_t sprstat_abandoned_sent;
};
sprstat_assoc_id: This parameter is ignored for one-to-one style
sockets. For one-to-many style sockets, this parameter indicates
for which association the user wants the information. It is an
error to use SCTP_{CURRENT|ALL|FUTURE}_ASSOC in sprstat_assoc_id.
sprstat_sid: This parameter is ignored.
sprstat_policy: This parameter indicates for which PR-SCTP policy
the user wants the information. It is an error to use
SCTP_PR_SCTP_NONE in sprstat_policy. If SCTP_PR_SCTP_ALL is used,
the counters provided are aggregated over all supported policies.
sprstat_abandoned_unsent: The number of user messages that have been
abandoned using the policy specified in sprstat_policy for the
association specified by sprstat_assoc_id, before any part of the
user message could be sent.
sprstat_abandoned_sent: The number of user messages that have been
abandoned using the policy specified in sprstat_policy for the
association specified by sprstat_assoc_id, after a part of the
user message has been sent.

There are separate counters for unsent and sent user messages because
the SCTP_SEND_FAILED_EVENT supports a similar differentiation.
Please note that an abandoned large user message requiring SCTP-level
fragmentation is reported in the sprstat_abandoned_sent counter as
soon as at least one fragment of it has been sent. Therefore, each
abandoned user message is counted in either sprstat_abandoned_unsent
or sprstat_abandoned_sent.
If more detailed information about abandoned user messages is
required, the usage of the option described in Section 4.3 or the
subscription to the SCTP_SEND_FAILED_EVENT is recommended.
sctp_opt_info() needs to be extended to support SCTP_PR_ASSOC_STATUS.
4.5. Socket Option for Getting and Setting the PR-SCTP Support
(SCTP_PR_SUPPORTED)
This socket option allows the enabling or disabling of the
negotiation of PR-SCTP support for future associations. For existing
associations, it allows one to query whether or not PR-SCTP support
was negotiated on a particular association.
Whether or not PR-SCTP is enabled by default is implementation
specific.
This socket option uses IPPROTO_SCTP as its level and
SCTP_PR_SUPPORTED as its name. It can be used with getsockopt() and
setsockopt(). The socket option value uses the following structure
defined in [RFC6458]:
struct sctp_assoc_value {
sctp_assoc_t assoc_id;
uint32_t assoc_value;
};
assoc_id: This parameter is ignored for one-to-one style sockets.
For one-to-many style sockets, this parameter indicates upon which
association the user is performing an action. The special
sctp_assoc_t SCTP_FUTURE_ASSOC can also be used; it is an error to
use SCTP_{CURRENT|ALL}_ASSOC in assoc_id.
assoc_value: A non-zero value encodes the enabling of PR-SCTP,
whereas a value of 0 encodes the disabling of PR-SCTP.
sctp_opt_info() needs to be extended to support SCTP_PR_SUPPORTED.